Gas sensing apparatus and method

ABSTRACT

A sensing apparatus and method for qualitatively and quantitatively analyzing small amounts of selected oxidizable gases in a carrier gas including the associated electronic circuitry is described. The apparatus preferably includes two semi-conductor type sensing elements in electrical parallel so that the difference in resistance of the two sensors can be detected, which can be of the N or P type and which decrease or increase in electrical resistance as a function of the concentration of an oxidizable gas. Also, a method for determining fruit maturity and ripeness by the detection of ethylene in the internal atmosphere of a fruit is described which preferably uses the apparatus with the semi-conductor type sensing elements, but can use other variable resistance type gas sensing means.

This application is a continuation-in-part of application Ser. No.29,293, filed Apr. 12, 1979, which is now abandoned.

SUMMARY OF THE INVENTION

The present invention relates to a gas sensing apparatus and method forqualitatively and quantitatively analyzing small amounts of selectedoxidizable gases in a carrier gas, particularly a method for thedetection of fruit maturity and ripeness using ethylene in the internalatmosphere of the fruit as a measure. In particular, an apparatus andmethod is described using a N or P type semi-conductor sensing element.

PRIOR ART

The prior art has described many different types of gas sensing means.The present invention is concerned with those sensing means havingsensing elements which increase or decrease in electrical resistance asa function of the concentration of small amounts of oxidizable gases ina carrier gas, usually in amounts much less than one percent (1%) byvolume.

The patent art contains detailed descriptions of variable resistancetype sensing means, particularly those having N or P semi-conductor typeelements. Included are U.S. Pat. No. 3,051,895 to Carson; U.S. Pat. Nos.3,603,954, 3,625,756, 3,631,436, 3,695,848, 3,644,795, 3,835,529 and3,900,815 all issued to Taguchi; U.S. Pat. No. 3,997,837 to Betz et al;U.S. Pat. No. 4,123,225 to Jones et al; U.S. Pat. No. 4,045,177 toMcNally and U.S. Pat. No. 4,129,030 to Dolan. Literature includesarticles by Mallard et al, Analytical Chemistry, Vol. 49, pages 1275 to1277 (July 1977), and by Lewart, Popular Electronics, pages 46 and 47(August, 1976). Also, commercially available gas sensing devices of theN semi-conductor type are described in literature of Figaro EngineeringLt of Osaka, Japan (May 1, 1976) and are preferred for the purposes ofthe present invention because of the linearity of decrease in resistanceas a function of increasing amounts of oxidizable gases and theirsensitivity when used in accordance with the present invention.

In the growing of fruits, such as apples, it is a difficult problem todetermine when the fruit should be harvested so that it can be storedwithout excessive ripening and deterioration in storage. Thus thedecisions of when to harvest a particular block of apples and how andfor how long to store them, are among the most important managementdecisions made by the fruit grower and storage operator. High qualityapples depend upon harvesting them within a certain specified period inrelation to how and when the fruits are marketed. For optimum eatingquality, apples marketed during the harvest season should be allowed toremain on the tree until nearly ripe. However, practical and widelyrecognized economic reasons largely rule this out. Fruits forrefrigerated and short-term controlled atmosphere (CA) storage of 4 to 6months should be harvested and stored after ripening has begun but priorto their attaining full eating quality. Ideally, fruits destined forCA-storage periods of 7 to 9 months or longer should be harvested andstored slightly before or just after the ripening process has begun.Storageability is inversely proportional to the degree of ripeness atharvest. Additionally, certain physiological disorders which develop instorage show a relationship to fruit ripeness at harvest. Generallyspeaking, apples harvested 7 to 10 days or more before they haveattained full maturity are susceptible to scald and bitter pit and areprone to shrivel. Furthermore, the apples are likely to be of lowdessert quality from the standpoint of color, flavor and texture. Fruitsize is also sacrificed by harvesting too early. However, applesharvested after attaining full ripeness on the tree are subject tophysiological disorders such as core browning, internal breakdown,mealiness and Jonathan spot, which are associated with senescence.Moreover, ripe fruit are prone to bruising and decay. Recognition of thestage of ripeness of apples after harvest is therefore more importantthan it is before harvest. The potential storage life of the fruit islargely determined at the moment the fruit are removed from the tree.Delays in cooling and other improper handling procedures further reducethe postharvest life.

The harvesting guidelines outlined above appear to be simple andstraightforward. However, while most people can recognize a "green" orunripe apple and one that is ripe, no one can tell, without the use ofelaborate electronic equipment precisely when ripening is initiated.

It is possible to estimate the optimum apple harvest dates for CAstorage in various areas. Michigan and other states have similarprograms in operation. Date of full bloom and temperature during thegrowing season are used. Predictive methods are of value, but it isimportant to be able to ascertain the actual progress of fruitmaturation and ripening by objective means.

Examination of the fundamental changes that occur in apples as theymature and ripen on the tree reveals that a number of physiologicalprocesses proceed culminating in several readily observablecharacteristics. These include softening of the flesh, loss ofchlorophyll, increase in carotenoid and anthocyanin pigments, conversionof starch to sugars, decrease in acidity and astringency and an increasein juiciness, flavor and aroma. These processes are brought aboutthrough qualitative and quantitative changes in metabolism. Qualitative,in that new biochemical pathways are initiated and quantitative, vis avis a several-fold increase in the rate of metabolism. Ethylene is theunderlying factor responsible for initiating the ripening process.

Ethylene is a gaseous plant hormone that causes fruits to ripen. It isproduced at a low and fairly constant rate as fruits develop on thetree. This low rate of production normally persists through the last fewweeks of development unless the fruits are perturbed by someenvironmental or chemical stress. The ethylene production rate thenincreases abruptly signalling the initiation of the ripening process,which culminates in the biophysical and biochemical changes notedearlier. The change in ethylene production rate is immense in proportionto the low steady-state rate typical of immature fruit; and over thecourse of 7 to 10 days, causes the internal atmosphere concentration toincrease from about 0.1 ppm to 10 or 100 ppm. This rapid increase inethylene concentration follows a predictable time course in which thelogarithm of the ethylene concentration increases linearly with timereaching a maximum when the apple is fully ripe. This rapid increase inethylene production is termed "autocatalytic" and is a prerequisite forfruit ripening.

Determination of the low ethylene levels (e.g. 0.1 ppm) within immaturefruits requires a very sensitive and elaborate gas chromatograph in thelaboratory. However, detection of higher ethylene levels (e.g. 0.5 ppmor higher) as fruits begin to ripen can be accomplished with lesselaborate equipment. Several years ago, the inventors developed amodification of the Kitagawa colorimetric procedure for measuringethylene and adapted it for use by fruit storage operators to assessfruit maturity and determine storageability of different lots of apples.This procedure entailed extracting the internal gaseous atmosphere froma bulk sample of 15 to 20 apples and analyzing the ethylenecolorimetrically. A chemical in the indicator tube changes color fromyellow to blue as it reacts with ethylene. The indicator tubes candetect an ethylene concentration of 0.5 ppm or greater in a 200 ml gassample. About 15 minutes is required for each sample. The glassware andequipment costs about $200 and the indicator tubes about $1 each.Several fruit storage operators are equipped for this analysis. It isexpensive and not sensitive enough. Ethylene causes ripening of otherfleshy fruits.

What is needed by the art is a relatively inexpensive, portable gassensing apparatus and method. Such a device would also be useful tomonitor ethylene evolved from the fruit or added to the atmosphere infruit storages. Such detection apparatus using variable resistance typesensing elements which are affected by oxidizable gases would also beuseful as a detector for ethyl alcohol in sobriety tests; to detectammonia leaks in refrigeration systems; carbon monoxide and methane inmines; engine exhausts; and furnace stack emissions; and in any othersetting where it is desired to know the presence and/or concentration ofa small amount of an oxidizable gas in a carrier gas.

OBJECTS

It is therefore an object of the present invention to provide a gasdetector apparatus and method for determining the presence of and theconcentration of selected oxidizable gases in a carrier gas. It isfurther an object of the present invention to provide an apparatus andmethod for determining the maturity and ripeness of fruits. Furtherstill, it is an object of the present invention to provide an apparatuswhich is portable, relatively inexpensive and simple to use in thefield, such as an orchard, warehouse storage, or in a vehicle. Further,it is an object of the present invention to provide an apparatus whichis useful in teaching and research. These and other objects will becomeincreasingly apparent by reference to the following description and tothe drawings.

IN THE DRAWINGS

FIG. 1 is an electrical diagram showing the preferred circuit of thepresent invention utilizing N-type semi-conductor elements in two gassensors S1 and S2 in electrical parallel so that the difference inresistance of the two sensors can be detected and which can be used todetermine the presence and the concentration of small amounts of anoxidizing gas in a carrier gas.

FIG. 2 is a front partial cross-sectional view of the apparatus of thepresent invention, particularly illustrating an adsorption or absorptiongas chromatography column between N-type semi-conducting elements insensors S1 and S2 in a gas manifold mounting block which also acts as aheat sink.

FIG. 3 is a front perspective view of the gas manifold mounting blockand column of FIG. 2 installed in a portable housing.

FIG. 4 is a front view of sensor S1 and a miniature 7-pin socket whichpresses onto terminals on the gas sensor S1 (or S2).

FIG. 5 is a plan view showing a commercial gas sensor S1 or S2 adaptedto provide gas flow across the sensing element mounted in a tube orhousing with a hole on the side of the tube as also shown in FIG. 2.

FIG. 6 is a schematic view of the sensor S1 or S2.

FIG. 7 is an improved detector unit including a sensor isolation circuitand a voltage amplifier circuit.

GENERAL DESCRIPTION OF THE INVENTION

The present invention relates to an electrically powered oxidizable gasdetector apparatus which comprises at least two gas sensor means eachhaving a sensing element which decreases or increases resistance as afunction of the adsorption of oxidizable gases in a carrier gas on thesensing elements in a parallel electronic circuit so that the differencein resistance of the two sensing elements can be detected when poweredby a direct current power source, wherein the difference in resistanceof the sensing elements is a function of the increasing concentration ofthe oxidizable gases and is detected by the electronic circuit andwherein one sensor element is adapted to be used for a reference gas andthe other for an unknown oxidizable gas; electrical heater meansadjacent to each sensing element to provide an operating temperature andpowered in parallel by the direct current source and controlled by avoltage regulator means to maintain a constant voltage to each heatermeans; and detection means in conjunction with the sensing elements fordetecting the difference in resistance between the sensing element forthe reference sensing element and the other sensing element when thereis a difference in composition of gaseous oxidizable compounds in thegases supplied to the two sensing means. The usual reference gas, air isalso the carrier gas.

In particular, the present invention relates to the improvement in anelectrical detector apparatus which comprises an electrically poweredgas sensor means in an electronic circuit with a heating element tosupply an operating temperature to at least one sensing element adjacentthereto wherein the sensing element decreases or increases in resistanceupon exposure to an oxidizable gas in a carrier gas, wherein the amountof decrease or increase in resistance of the sensing element is afunction of the increasing concentration of the oxidizable gas and isdetected by detection means in the electronic circuit and wherein thesensing element is mounted inside a tube as a sensor housing withopenings for inlet and exhaust of gases adjacent the sensing elementwherein the tube is closed at at least one end and has a small hole asone opening through the side of the tube adjacent the sensing element; agas manifold mounting block made of a heat conductive material, metal,for example, to act as a heat sink having an opening adapted to receivethe tube in closely spaced relation and having a first conduit leadingto the hole in the side of the tube and a second conduit through themounting block adjacent the other opening in the tube; and seal meansbetween the opening in the gas manifold mounting block and the gassensor tube to prevent gas leakage from the space between the tube andthe gas manifold mounting block and to provide a flow of the gasesthrough the openings and in the tube across the sensing element. Thehole in the sensor means tube is preferably between about 1 and 2 mm inaverage diameter and is positioned such that the gas is directed on thesensing element.

The improved gas sensing apparatus particularly increases thesensitivity of standard N semi-conductor type sensing elements,particularly those made of sintered tin oxide, by several multiples of10 on a reproducible basis which is a very unexpected result. The tinoxide elements have a usual sensing range of above about 0.01 percent or100 parts per million of an oxidizing gas in a carrier gas. Thissensitivity is increased by the apparatus of the present invention todown to about 0.2 parts per million. The P type semi-conductor typeelements are also useful.

The present invention generally relates to the method for determiningthe difference in concentration of small amounts of oxidizable gases ina carrier gas which comprises: providing at least two gas sensor meanseach having a heated sensing element which decreases or increasesresistance and thus increases or decreases current output at a constantvoltage as a function of the increasing adsorption of small amounts ofgaseous oxidizable compounds in a carrier gas on the elements in aparallel electronic circuit with a detection means so that thedifference in resistance between the two sensing elements can bedetected and is powered by a direct current power source, wherein onesensor element is for detecting a reference gas and the other fordetecting an oxidizable gas in a reference gas; and introducing thegases to the sensing elements in the sensor means and determining thedifference in resistance as current output from the sensing elementswith the detection means.

The present invention also relates to the method for determining theconcentration of small amounts of oxidizable gases in a carrier gaswhich comprises providing at least two gas sensor means each having aheated sensing element which decreases or increases resistance and thusincreases or decreases current output at a constant voltage as afunction of the increasing adsorption of small amounts of gaseousoxidizable compounds in a carrier gas on the sensing element in aparallel electronic circuit so that the difference in resistance betweenthe two sensing elements can be detected and is powered by a directcurrent power source, wherein the sensor means are connected by aconduit in a sealed flow path at opposite ends of the conduit with anupstream and a downstream sensor means; introducing a reference gasthrough the upstream sensor means and providing an oxidizable gas in areference gas in the conduit adjacent the upstream sensor means suchthat the downstream sensor means detects the concentration of theoxidizable gas as it flows across the element and provides a responsefrom the electronic circuit which is proportional to the concentrationof the oxidizable gas. The conduit is preferably filled with anadsorptive or absorptive material for selected oxidizable gas or gases.Such materials include activated alumina, silica gel and othercommercially available column packing gas chromatographic materials.Such materials provide a separation in time of multiple compounds in acarrier gas.

The present invention particularly relates to the method for determiningfleshy fruit maturity and ripeness particularly prior to harvest, afterharvest, or during storage which comprises providing an electricallypowered gas sensor means which decreases or increases resistance uponcontact with an oxidizable gas in air, wherein the amount of decrease orincrease in resistance of the sensing element is a function of theincreasing concentration of the oxidizable gas, such as ethylene, and isdetected by an electronic circuit whose current output is a function ofthe difference in resistance of the sensing elements; selecting animmature fleshy fruit and a maturing fleshy fruit of the same specieswith air in the internal atmosphere of the fruits which containsincreasing concentrations of at least one specific oxidizable gas as aresult of maturation and ripening; and separately introducing airsamples removed from the internal atmosphere of the immature fruit andthe maturing fruit of the same species across the gas sensing elementand determining the difference in the current output from the electroniccircuit, whereby as the maturing fruit begins to ripen the concentrationof the specific oxidizable compound, such as ethylene, increasesindicating that fruit ripening processes are proceeding and that thefruit is becoming ripe and wherein the remaining oxidizable compounds inthe air samples remain relatively constant as between the maturing andimmature fruits. Other oxidizable compounds are formed in fruits asripening proceeds but usually only after the ethylene concentration hasreached levels readily detectable by the apparatus described herein.

SPECIFIC DESCRIPTION

Referring to the electrical circuit of FIG. 1, sensors S1 and S2 with Ntype semi-conductor sensing elements 10 and 11 are shown connected inelectrical parallel. The elements are powered by a 12.6 volt battery 13(or from an AC to DC regulated power supply) having the positiveterminal leading to the sensing elements 10 and 11 through a diode D4(which protects the circuit from reverse current flow), miniature singlepole, single throw switches 14 and 15 and resistance R2 to the junction16 between the elements 10 and 11. The other side of the elements 10 and11 is connected to adjustable resistances R5 and R7, respectively, whichare each connected to ground G. A detector means 17 shown by brokenlines forms an electrical bridge between resistances R5 and R7 and thesensors 10 and 11. Thus the basic circuit is similar to a Wheatstonebridge which is well known to those skilled in the art.

The detector means 17 (shown in broken lines) includes a fixed resistorR3, variable resistance R4, a microammeter M1 and a diode D2 to limitthe direction of current flow. The resistance R3 protects againstoverload of the meter M1 and R4 provides a means for adjusting output toexternal recorder means. Between the switches 14 and 15, a lead 18 isprovided through resistance R1 through light emitting diode D1 to groundG which detects current flow to the voltage regulator 19 through lead18. The voltage regulator 19 is provided with common outlet leads 20 and21 to heater elements 22 and 23 for sensing elements 10 and 11 which arein turn connected to ground G. The controlled voltage from the voltageregulator 19 is necessary to provide a controlled uniform heating of theelements 10 and 11. A capacitor C1 is provided between ground G andleads 20 and 21 to smooth any minor voltage surges from the regulator19. At point 24 a lead 26 to ground is provided through a light emittingdiode D3 to indicate that power is being supplied to the sensingelements 10 and 11. A heater 25 connected to ground is provided forheating the gas manifold mounting block 100 (shown in dotted lines inFIG. 1) for the sensors 10 and 11. Terminals 27 are provided for arecorder means 28 which can be used in addition to meter M1.

Referring to FIG. 2, the gas manifold block 100 is made of metal,preferably brass, to provide a common heat sink for the sensor means S1and S2. This construction is important to the reliability andsensitivity of the measurements. The gas manifold mounting block 100 ispreferably heated by external heater 25 to a fixed temperature, normallybetween about 20° and 50° C., preferably 35° C. Since the apparatus isto be portable and used in all sorts of environments the heater 25 isimportant.

Both sides of the block 100 as shown in FIG. 2 have a number of elementsin common. The block 100 is provided with two (2) cylindrical holes 101for close fitting of the sensors S1 and S2 with grooves 103 midway intothe holes 101 supporting ring seals 104. At the bottoms 105 of the holes101, seats 106 are provided with ring seals 107. Between the seals 104and 107 and the sensors S1 and S2 are annular sealed spaces 108. Spaces102 are also provided between the sensors S1 and S2 and the bottom ofthe holes 101. The gas manifold mounting block 100 is provided withholes 109 concentric with the longitudinal axis of the holes 101 leadingto the bottoms 105 of the holes 101. The holes 109 are provided withthreaded openings 110. An outlet fitting 111 is provided in one of theopenings 110 and an inlet fitting 112 is provided on the other opening110.

Perpendicular to the axis of the holes 101 are provided holes 113 and114 from opposite sides 115 and 116 of the gas manifold mounting block100. These holes 113 and 114 are provided with threaded openings 117 and118 which are concentric with the holes 113 and 114. The holes 113 and114 with openings 117 and 118 are mirror images of each other. At oneend 115 of the gas manifold mounting block 100, is a right angle elbow119 with one end 119a threaded into hole 117. The other threaded end119b is fitted with a nut 120 mounted on an extension 121a of a flangedtube 121 to provide a gas sealed connection to the elbow 119. The tube121 is in the form of a helical coil with a second flanged extension121b fitted to a second nut 122 attached to a threaded extension 123a ofa T 123 having a leg 123b threaded into the opening 118 in the block100. The tube 121 is filled with an oxidizable gas adsorptive orabsorptive material and acts as a gas chromatography column. A threadedinlet leg 123c is fitted with a nut 124 having a sealed septum 125 (FIG.3) which is penetrable by a syringe 300 and needle 301 which is aninjection method well known to those skilled in the art. As seen in FIG.3, the gas manifold block 100 is mounted in a housing 126 by means ofscrews 127. A handle 128 is provided on the housing 126 for carrying.The output terminals 27 from the detection means 17 can be fed to arecorder 28 or other device.

The details of the construction of the preferred gas sensor S1 and S2which are identical are shown in FIGS. 2, 4, 5 and 6. A hollowcylindrical housing or tube 200 made of an inert plastic is providedwith a small hole 201 leading to the space 108. The housing 200 has ahollow interior 202 in which is mounted a N-type gas sensing element 203which is schematically shown as sensing element 10 or 11 in FIGS. 1 and6. The hole 201 is adjacent the sensing element 203. A screen 204 andopening 205 are provided on one side of the sensor S1 and S2. Thesensors S1 and S2 have a second opening 206 covered by a screen 207.This is the conventional construction of a Taguchi type gas sensor (TGS)except for the hole 201 which is important for flow direction of theoxidizable gases across the elements 10 and 11. The elements S1 and S2have a strip 208 closing the opening 206. The opening 206 is sealed withthe metal strip 208 using an epoxy resin bonding agent or the like. Theopenings 201 of the sensors S1 and S2 are connected to holes 113 and114. The sensors S1 and S2 shown in FIGS. 2, 4, 5 and 6 are a TaguchiGas Sensor Figaro 812. The important aspect of the modification is theflow path of the gas across the sensing element 203.

FIG. 4 shows a conventional miniature 7-pin socket 129 which bolts tothe block 100 by means of bolts 130. The leads 1a, 2a and 3a correspondto terminals 1, 2 and 3. The terminals 1 to 6 project from the sensorhousing 200 and are electrically connected as shown in FIG. 6. Only twoof the terminals 1 and 4 or 3 and 6 are connected into the circuit asshown in FIG. 1.

Table 1 shows the functional characteristics of the Figaro_(TM) SensorNo. 812

                  TABLE 1                                                         ______________________________________                                        Heater Volts         5.0 ± 0.2V                                            Heater Power Consumption                                                                           620 mW                                                   R.sub.(IB 1000)      1kΩ-10kΩ                                     This represents sensor resistance when exposed                                to 1000 ppm isobutane in air                                                  R.sub.(IB 3000) /R.sub.(IB 1000)                                                                   approx. 0.55                                             This represents ratio of                                                      sensor resistance in 3000 ppm isobutane                                       sensor resistance in 1000 ppm isobutane.                                      Warm-up Time         within 2 minutes                                         ______________________________________                                    

This sensor means is designed as a general purpose oxidizable gas sensormeans in the concentration range between about 0.05 to 1.0 percent in acarrier gas.

Table 2 shows the elements of the preferred circuit shown in FIG. 1.

                  TABLE 2                                                         ______________________________________                                        1Cl            MLM 309K Regulator                                             Cl             1μ F electrolytic capacitor                                 D1&D3          light emitting diode                                           R1, R2, R6     220 ohm (1/2 Watt)                                             R3             2.2 K (1/2 Watt)                                               R4             1 kilo ohms linear taper Pot.                                  R5             50 kilo ohms linear taper Pot.                                 M1             0-50μ Amp Meter                                             D2             1N904 diode                                                    D4             Mallory.sub.™  M2.5A diode                                  ______________________________________                                    

The TGS gas sensors can detect:

(1) hydrocarbons and their derivatives such as: methane; ethane;propane; butane; pentane; hexane; heptane; octane; decane; petroleumether; petroleum benzene; gasoline; kerosene; petroleum naphtha;acetylene; ethylene; propylene; butadiene; butylene; benzene; toluene;o-xylene; m-xylene and ethylene oxide;

(2) halogenized hydrocarbons such as: methyl chloride; methylenechloride; ethyl chloride; ethylene chloride; ethylidene chloride;trichloroethane; vinylidene chloride; trichloro ethylene; methyl bromideand vinyl chloride;

(3) alcohols such as: methanol; ethanol; n-propanol; iso-propanol;n-butanol and iso-butanol;

(4) ethers such as: methyl ether and ethyl ether;

(5) ketones such as: acetone and methyl ethyl ketone;

(6) esters such as: methyl acetate; ethyl acetate; n-propyl acetate;isopropyl acetate; n-butyl acetate and iso-butyl acetate;

(7) nitrogen compounds such as: nitro methane; mono methyl amine;dimethylamine; trimethylamine; mono ethylamine and diethylamine;

(8) inorganic gases such as: ammonia; carbon monoxide; hydrogen;hydrogen cyanide; hydrogen sulfide and carbon disulfide.

OPERATION

The operation of the gas sensor apparatus for the detection of ethylenein air from the internal atmosphere of a fleshy fruit is generallyillustrative. Ethylene is initially present in low amounts in unripefruits (between about 0.05 to 0.1 ppm) and increases 100 to 1000 timesas it causes fruit to ripen. The problem is to detect the precise pointat which the ethylene concentration increases to about 0.2 and 0.5 ppmindicating the point of maximum storability of the fruit and the abilityto ripen in storage. This increase in ethylene initiates autocatalyticethylene production and fruit ripening. As the ripening processproceeds, numerous aldehydes and ketones develop which gives the fruitthe characteristic "fruit" smell and flavor. These gases can also besensed by the apparatus of the present invention which detectsoxidizable gases quantitatively down to about 0.1 to 0.2 ppm (1 to2×10⁻⁵ %).

In one mode of operation of the apparatus of FIGS. 1 and 2, samples ofthe internal atmospheres from the fruit of an early maturing variety anda much later maturing variety at the same temperature are removed with asyringe 300 by means of a needle 301. This step compensates for humidityand thermal effects. The plug 131 and nut 124 are removed in this modeof operation. The samples are then introduced through fittings 111 and112 across the elements 201 of sensors S1 and S2 and out throughconduits 113 and 114 to the atmosphere. The later maturing varietysample is passed across S1. The "background gases", are essentially thesame in both varieties and thus the early ripening variety will show apositive response for ethylene in S1 as read on the meter M1 if thefruit is beginning to ripen. Unexpectedly it has been found that thismethod can be used with many different early and late maturing varietiesof fruits.

The use of the apparatus for apples is particularly illustrative. Inethylene detection mode of operation, preferably two 1 ml gas samples insyringes are injected simultaneously into the two sensors S1 and S2. Onesample is from the fruit of questionable ethylene content and the otherfrom the fruit of a later maturing variety with low ethylene level,which serves as a blank or control. If the gas sample in question doesnot contain ethylene, the detector meter M1 does not respond. Ifethylene is present, the meter responds immediately and is linear from 0to at least 10 ppm of ethylene. The analysis takes about 3 minutes persample.

In the ethylene determination mode, a 1 ml gas sample from the fruit inquestion is injected in port 125 and is carried through the column 121to sensor S2, and the analysis is complete in about 2 minutes. The meterM1 response is linear from 0 to at least 10 ppm of ethylene.

Ethylene, is quantitatively determined using the gas chromatographiccolumn of activated alumina in tube 121 as shown in FIG. 2. Thisactivated alumina column has an affinity for ethylene but not aldehydes,esters and alcohols. The plug 131 and nut 124 are in place as shown inFIG. 2. Thus a syringe 300 and needle 301 is used to inject a gas samplefrom the internal atmosphere (or seed cavity) of the maturing fruit intoseptum 125 so that it is inside T 123. A carrier gas, preferably air, atabout 20 psig (2.4 atm) or from an aquarium type air pump (about 2 psig1.1 atm) by using a chromatography column of appropriate resistancegives a carrier gas flow rate of about 8 to 15 ml per minute. Thecarrier gas is introduced into fitting 112 and across the element 203 ofelement S2 and then the sample is carried through tube 121 to sensor S1where the element 201 of S1 responds by precisely decreasing inresistance as a function of the ethylene concentration. The downstreamsensor S1 gas pressure is above atmospheric pressure and the upstreamsensor S2 has a pressure above that of S1. The upstream sensor S2 has apressure in space 202 which is above atmospheric and above the pressurein the space 202 in S1. The difference in resistance of sensors S1 andS2 is displayed by the meter M1. The unit is adjusted using the carriergas, R5 and R7 to make certain the meter M1 is at a reference or "0"point. The gas chromatography tube 121 is preferably filled withactivated alumina which separates ethylene from ethane, aldehydes,ketones, alcohols and esters present in the internal atmospheres offruits. Most of these other oxidizable materials are passed through thecolumn in about 5 seconds while ethylene is delayed for about 30 secondsto 2 minutes.

Calibration of the meter M1 response requires an accurately determinedand standardized mixture of ethylene at a concentration between 5 and 10ppm in air. Since the meter M1 response is linear with ethyleneconcentration, calculation of ethylene in samples is quite simple whenusing a fixed injection volume. A numerical factor (expressing ppmethylene in the standard gas/meter units developed by the standardsample) is multiplied by sample meter units developed by the unknownsample, and this yields ppm of ethylene in the sample. For example, if a1 ml sample of a 10 ppm ethylene standard gives 40 micro-amps, then theconcentration of ethylene in the sample is calculated as follows:##EQU1##

Samples of the internal atmosphere of apple fruits can be readilyobtained after harvest or while the fruits are on the tree. Sampling isdone by withdrawing 1 ml of gas from the internal cavity with a needle301 (18 or 20 gauge; 11/2") and syringe 300 by entering the calyx end ofthe apple. A clean-out wire should be used to prevent clogging of theneedle 301 with tissue. A composite sample from several fruits can beobtained by employing a 10 to 20 ml syringe 300. Withdrawing a 1 ml gassample causes a slight vacuum within the fruit, and this is equalized asair enters through the open lenticels over the fruit surface. Thisresults in a negligible (less than 1%) dilution of the gas sample. Atmost this would be about 3% based on instantaneous dilution.

It will be appreciated that other resistance type sensors can be usedfor the fruit maturity and ripeness detection such as the catalyticdetectors of the prior art. Further many different absorption oradsorption materials can be used in the gas chromatography column. Manydifferent kinds of fruits both edible and unedible as well as flavorscan be tested using the method of the present invention. Other types ofdetecting means can be used including recorders which trace a responseon paper as a function of concentration or light emitting diodes or adigital display which respond as a function of concentration ranges. Thecolumn can be replaced by or be in series with a container or chamber orroom containing the oxidizable gas to be detected and/or the source ofthe gas such as a fruit. Examples are: a banana ripening room, a tomatoripening room, a tobacco ripening room (ethanol to ethylene). The outputof the sensors S1 and S2 can also be used to electrically regulate acontrol means such as a valve or a fan. The apparatus can also be usedto monitor atmospheres in ethylene ripening rooms, such as for bananas,tomatoes and citrus fruits or to detect the presence of undesirableethylene levels such as in flower warehouses or other storages or intransportation vehicles. The apparatus can also be used for onstreamdetection wherein the column contains a material for removal of a gasafter being detected by a first detector and wherein the downstreamdetector indicates the absence of the removed gas by the difference inresistance from the sensors S1 and S2. All of these variations will beobvious to those skilled in the art.

FIG. 7 shows an improved detector unit circuit including, in addition tothe sensor means S₁ and S₂, a sensor isolation circuit means 400 andvoltage amplifier circuit means 500. The values of the resistances "R"and the identification of other elements are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        R24           2.2 kilo ohms                                                   R26           50 kilo ohms variable                                           R27           43 kilo ohms                                                    R28 and R29   22 kilo ohms                                                    R30 and R31   1.0 megohm                                                      R32           100 kilo ohm                                                    R33           33 kilo ohm                                                     R34 and R36   10 kilohm variable                                              R35           100 ohm                                                         R37           2.2 kilo ohm                                                    R38 10 kilo ohms variable                                                     R39           100 ohm                                                         D.sub.5       diode                                                           LM.sub.1 and LM.sub.2                                                                       1/2LM 358 (National semiconductor)                                            dual operational amplifier                                      Power         12 volts DC                                                     M.sub.2       milliammeter                                                    ______________________________________                                    

LM₁ and R28 through R31 function as a voltage follower which isolatesthe sensors S₁ and S₂ from the rest of the circuits. The output of LM₁is fed into LM₂ which functions as a linear amplifier. The gain of LM₂depends on the setting of S401, which functions as a range selector. Inthe 10 ppm range, the gain of LM₂ is about 3.3. In the 1 ppm range, thegain of LM₂ is exactly 10 times that of 10 ppm range. R₃₄ is used tofine tune the gain of LM₂ at the 1 ppm range. R₃₆ functions as an offsetadjustor which nulls the output of LM₂ at the 1 ppm range. The voltageoutput of LM₂ is fed to a milliammeter M₂ to display the response of thesensors S₁ and S₂.

The improved circuit of FIG. 7 is useful for fruit maturity and ripenessdetection as described previously. It has also been found to beparticularly useful for quantitative carbon monoxide detection in thecirculatory or pulmonary system of mammals, particularly humans. Theimproved circuit will accurately detect between about 0.5 and 500 ppm ofcarbon monoxide.

OPERATION FOR CARBON MONOXIDE DETECTION

Carbon monoxide poisoning represents a major health problem for humansand is difficult to detect clinically and quantitatively. There has beenno means for rapidly determining lung air or blood concentrations ofcarbon monoxide particularly under emergency conditions. Presently flameionization detection (FID) is used by reducing carbon monoxide tomethane; however, this test is difficult to perform accurately, and isexpensive and time consuming. Methane may be a natural component ofpulmonary air which interferes with the detection of carbon monoxideunless the gases are separated by a chromatography column.

The circuit of FIG. 7 was used with the device shown in FIG. 2 forcarbon monoxide detection. Air is sampled from the lungs by breathing orforcing air from the lungs into a syringe 302 via a mouth piece andaerosol hose sampling device. Pulmonary air at the end of exhaling isused for the lung air sample since it represents the air mixture inequilibrium with the alveoli and would therefore be most reliable forcarbon monoxide detection. Dried air or other carrier gas is injectedthrough inlet tube 112 over sensor S₂ and then into column 121. Thesample is injected through septum 125, is carried through the column 121by the air across the active sensor S₁ and the ppm of carbon monoxide isread on the meter M₂. The column 121 contains a 5 angstrom molecularsieve which chromotographically separates oxidizable gases in thesample, particularly methane and carbon monoxide. The carbon monoxide isdetected in about 3 minutes.

For blood samples for carbon monoxide detection a sample of arterialblood is obtained via a syringe and is prepared for analysis accordingto the procedure of Collison et al., (Blood 14(2): 162-171 1968). Theblood derived gas sample is injected through septum 125, passed throughcolumn 121 and passed over the sensor S₁ by air which is first passedover the sensor S₂. The meter M₂ reading is translated into carbonmonoxide in the blood and is expressed as cubic centimeters of carbonmonoxide per 100 cubic centimeters of blood or as carboxyhemoglobin(HbCO) saturation. A normal value is about 1% HbCO for a non-smoker andabout 5% HbCO for a habitual smoker. The normal range of carbon monoxidein expired air is 1 to 5 ppm for a non-smoker and is 5 to 30 ppm forsmokers. Carbon monoxide poisoning produces much higher levels of 40 to60% HbCO saturation in the blood and correspondingly higher carbonmonoxide levels in the expired air. Atmospheric air commonly contains 1to 5 ppm carbon monoxide and in high traffic areas values in excess of50 ppm of carbon monoxide are frequently found.

We claim:
 1. An electrically powered oxidizable gas detector apparatuswhich comprises:(a) at least two gas sensor means each having a sensingelement which decreases or increases resistance as a function of theadsorption of gaseous oxidizable compounds in air on the sensingelements in a parallel electronic circuit so that the difference inresistance of the two sensing elements can be detected when powered by adirect current power source, wherein the difference in resistance of thesensing elements is a function of the concentration of the compounds andis detected by the electronic circuit and wherein one sensor element isadapted to be used for a reference gas and the other for an unknownoxidizable gas in the reference gas and wherein the oxidizable gas flowin at least the oxidizable gas sensor means is across the sensingelement in one direction and out of the sensor means in anotherdirection; (b) electrical heater means adjacent to each sensing elementto provide a common operating temperature and powered in parallel by thedirect current source and controlled by a voltage regulator means tomaintain constant voltage to each heater means; (c) detection means inconjunction with the sensing elements for detecting the difference inresistance between the sensing element for the reference sensing elementand the other sensing element when there is a difference in compositionof gaseous oxidizable compounds in carrier gases supplied to the twosensing means; (d) tube means providing a sealed flow path between thesensor means to the sensing elements; and (e) inlet means in the tubemeans for introducing an oxidizable gas in a carrier gas between thesensing means, wherein concentrations of ethylene as the oxidizable gasin the carrier gas between 0.1 and 10 ppm can be detected by thedifference in the resistance of the sensing elements.
 2. The apparatusof claim 1 wherein the sensing element is a N-type semi-conductor. 3.The apparatus of claim 2 wherein the sensing element is a sintered tinoxide N-type semi-conductor.
 4. The apparatus of claim 1 wherein twovariable resistance means are provided in the electronic circuit forinitially balancing the current output resulting from the difference inresistance of the two sensing elements while both of the sensingelements are being purged with a carrier gas.
 5. The apparatus of claim1 wherein an upstream and a downstream gas sensor means are provided ateach end of a gas chromatography column in the tube means containing aselectively gas absorptive or adsorptive material with an affinitytowards specific oxidizable gases such that an upstream gas sensor canbe purged with air acting as a carrier gas while an unknown gascontaining the oxidizable gas in the carrier gas is introduced in theinlet means between the upstream and downstream sensors in the columnand then the oxidizable gas is separated from other gases on the columnand then released in a short period of time and then is detected by thedownstream sensor means as the carrier gas and the oxidizable gas emergefrom the column.
 6. The apparatus of claim 5 wherein the sensor meanshas a sintered tin oxide N-type semi-conductor sensing element.
 7. Theapparatus of claim 5 wherein the chromatographic column material adsorbsor absorbs and then releases ethylene while permitting alcohols,aldehydes and esters obtained from the internal atmosphere of fruits topass through the column, thereby permitting qualitative and quantitativeanalysis of ethylene.
 8. The apparatus of claim 5 wherein the gas in thesensing element in the upstream sensor means is at a pressure above thatin the downstream sensor means.
 9. The apparatus of claim 7 wherein atleast the sensor means for the oxidizable gas in the reference gas isconstructed as a tube closed at one end and open at an opposite end andcontaining the sensing element mounted inside the tube with a small holeadjacent the sensing element through a side of the tube such that anoxidizable gas in a carrier gas can be passed across the sensingelement.
 10. The apparatus of claim 9 wherein the sensor means has asintered tin oxide N-type semi-conductor type sensing element.
 11. Theapparatus of claim 1 wherein the electronic circuit includes a voltageamplifier means in the circuit responsive to the current output from thesensing elements which increases the sensitivity of the detection meansto the oxidizable gas.
 12. In an electrical detector apparatus theimprovement which comprises:(a) at least two electrically powered gassensor means each in an electronic circuit with a heating element tosupply an operating temperature to a sensing element adjacent theretowherein the sensing element decreases or increases in resistance uponexposure to an oxidizable gas in a carrier gas, wherein the amount ofdecrease or increase in resistance of the sensing element is a functionof the concentration of the oxidizable gas and is detected by detectionmeans in the electronic circuit and wherein each sensing element ismounted inside a tube as a housing for the sensor means having opposingends, wherein the tube is closed at one end and open at the other endand has a small diameter hole as an opening for carrier and oxidizablegas inlet through a side of the tube adjacent the sensing element andwherein carrier and oxidizable gas flow is through the hole and acrossthe sensing element and out the open end of the tube; (b) a gas manifoldmounting block made of a heat conductive material to act as a heat sinkhaving an opening adapted to receive the tube in closely spaced relationand having a first conduit leading to the hole in the side of the tubeand a second conduit through the gas manifold mounting block adjacentthe open end of the tube, wherein each tube housing is mounted withinthe gas manifold mounting block; (c) seal means between the opening inthe gas manifold mounting block and the gas sensor tube to prevent gasleakage from the space between the tube and gas manifold mounting blockand to provide a flow of the gases through the tube across the sensingelement; (d) connector means providing a sealed flow path between thefirst and second conduits in the mounting block and holes in the sensingmeans to the sensing elements; and (e) inlet means in the tube means forintroducing an oxidizable gas in a carrier gas between the sensingmeans, wherein the concentrations of ethylene as the oxidizable gas inthe carrier gas between 0.1 and 10 ppm can be detected by the differencein resistance of the sensing elements.
 13. The apparatus of claim 12wherein the tube which is the sensor housing has a cylindricalcross-section and the seal means is a ring seal adjacent the firstconduit to the gas manifold mounting block opening and a ring sealadjacent the hole in the tube and second conduit such that gas can beexhausted through the block.
 14. A method for determining theconcentration of small amounts of carbon monoxide with other oxidizablegases in a carrier gas which comprises:(a) providing a sealed flow pathwith identical P or N type semi-conductor heated sensing elementstherein which decrease or increase in resistance as a function of theincreasing absorption of small amounts of the carbon monoxide in acarrier stream on the sensing elements; (b) introducing a carrier gasstream into said sealed flow path whereby the carrier gas enters a firstzone containing a first one of said sensing elements in a firstdirection in said first zone and exits said zone of the first detectorin a direction perpendicular to the direction of entry; (c) removingsaid sensed carrier gas stream and injecting therein a second gas streamcontaining the carbon monoxide having a concentration between 0.5 and500 ppm thereby forming a combined gas stream to be sensed; (d)conveying said combined gas stream through a conduit which directlyconnects and communicates said first and second identical sensingelements and said first zone with a second zone and forms the sealedflow path, wherein the conduit contains means for separating carbonmonoxide from other oxidizable gases in the combined gas stream so thatcarbon monoxide remains in the combined gas stream; (e) introducing thecombined gas stream into said second zone containing the secondidentical sensing element in a first direction; (f) contacting thecombined gas stream with said second sensing element and removing thesensed combined gas stream from said second zone along a directionperpendicular to the direction of entry; (g) sensing the resistance ofthe respective sensors in the first and second zones; and (h)determining the difference in resistance of the respective sensors as acurrent output as an indication of the oxidizable gas to be detected,wherein concentrations of ethylene as the oxidizable component in thecarrier gas between 0.1 and 10 ppm can be detected.
 15. The method ofclaim 14 wherein the carbon monoxide is air as the carrier gas streamfrom the circulatory or pulmonary system of a mammal.